In designing mobile terminals, such as mobile phones or cellular phones, improvement of the transmission efficiency of wireless communications is one of the points of greatest concern. A portion of the transmit power is lost due to human bodies located near the mobile terminal, and therefore, the transmission efficiency is reduced. Not only human bodies, but also arbitrary objects and articles that can prevent propagation of radio waves cause the transmission efficiency to decrease.
It is known that the transmission efficiency can be improved by simultaneously feeding power to multiple antenna elements whose amplitudes and phases are appropriately adjusted so as to control the directivity of the radio wave to be transmitted. A combination of the relative amplitude ratio and the phase difference between antenna elements is referred to as “weighting” or “feed weighting”. Such a technique is disclosed in, for example, Nishikido, et al., “Parallel 2-element Distributed Feed Antenna for Portable Wireless Equipment”, IEICE Society Conference B-1-190, 2003, and Yamaguchi and Uebayashi, “Improvement of Efficiency of Mobile Handsets by Distributed Feed”, IEICE Society Conference B-4-4, 2003.
In the conventional method for determining a weighting parameter, an initial weighting parameter (representing a combination of a relative amplitude ratio and a phase difference) is given to each of the antenna elements of a mobile terminal. Then, a radio wave is transmitted using all of the antenna elements, and the radiation efficiency is calculated and/or measured. Then, the weighting parameter for each of the antenna elements is updated based on the obtained radiation efficiency, a radio wave is transmitted again from all of the antenna elements, and the radiation efficiency (SAR) is calculated and/or measured. Subsequently, the radiation efficiency is calculated and/or measured for all the combinations of amplitude ratio and phase difference. The optimum set of weighting parameters that gives the optimum radiation efficiency is selected for the antenna elements of the mobile terminal.
However, to determine weighting parameters using the conventional method, the phase and the amplitude have to be changed successively, and the radiation efficiency has to be calculated and/or measured each time the weighting parameter is updated so as to cover all the possible combinations. This method can hardly be said to be an efficient method because the effort and labor required in calculation and measurement are considerably large. In general, it is advantageous to increase the number of antenna elements from the viewpoint of improvement of the directivity. However, as the number of antenna elements increases, the workload for computation greatly increases, and the weighting parameters cannot be determined quickly. For example, assuming that there are ten values of amplitude ratio between two antenna elements, ranging from 0.1 to 1.0 at stepsize of 0.1, and that there are 360 values of phase difference, ranging from 0 degrees to 360 degrees at stepsize of 1 degree, then there are 3600 combinations (i.e., the possible weighting parameters) for only two antenna elements. If three antenna elements are used, the number of weighting parameter becomes 3600×3600. For N antenna elements, there are 3600^(N−1) combinations. The computational workload increases in proportion to the power of (N−1).